I've done this mix for some rookies to get them used to co-ordinated turns...

In the sim what you do is assign another input to the receiver channel. This is simple....

Select the channel assigned to the rudder ( channel 4 ), then ADD another ( second ) complex input.
For the input source use the aileron input.

Modify the curve so that it goes to -8 at full left aileron to zero at stick center, then +8 again at full right aileron.

Check to to see that the rudder moves a bit as your ailerons go to their extremes, and that the rudder is moving in the right directly... if it is moving opposite of where it should, reverse the curve vaules.

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Re: Simulators

Try doing this kind of thing in other simulators and you'll find it is either impossible or convoluted... in Realflight it is almost trivial.

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I agree with Jester's suggestion that you LEARN to use the rudder instead of relying upon the electronics.